Optimizing Biochar Application Rates to Improve Soil Properties and Crop Growth in Saline–Alkali Soil

There is great demand for the amelioration of saline–alkali soils, which requires efficient and economical amendments. Biochar addition could alleviate the adverse impacts of saline–alkali stress in crops. However, their efficiency and optimal amounts in saline–alkali soil restoration remain contradictory and inconclusive. The objective of this study was to investigate the effects of biochar application on the properties of saline–alkali soil and crop growth, as well as to determine the optimal application rate of biochar. We conducted pot experiments with biochar (B) application rates, including 0 (CK), 1% (B-1%), 2.5% (B-2.5%), 5% (B-5%), and 10% (B-10%), studying the impact of biochar on soil water content (SWC), soil salinity, soil electrical conductivity (EC), soil ion content, soil nutrients, soil enzyme activity, and crop growth. A four-parameter Gaussian function was established for the curves depicting the relationship between soil salinity characteristics and the biochar application rates to determine the most optimal application rate. The results indicated that: (1) Compared to the CK, all biochar treatments improved soil water-holding capacity and reduced soil Na+ content and sodium adsorption ratio (SAR). (2) B-1%, B-2.5%, and B-5% treatments reduced soil content, EC, Cl−, and SO42− content over CK, while the results were reversed for the B-10% treatment. (3) Compared to the CK, all biochar treatments significantly increased soil fertility, enhanced soil enzyme activity (alkaline phosphatase, catalase, and urease activity), and significantly promoted the growth of maize. (4) The results of the Gaussian model suggested that a biochar application rate of 3.16% is the optimal rate for alleviating soil salinity in saline–alkali soils. This research demonstrated the potential of biochar to improve soil properties and promote crop growth and provided useful information on biochar application rates for ameliorating saline–alkali soils.